1. Field of the Invention
This invention relates to production of methane from subterranean coal beds, and more particularly to a process in which a carbon dioxide-containing gas is continuously injected into one or more injection wells to produce methane from one or more recovery wells spaced from the injection wells. The produced methane includes both free methane displaced by the injection gas and methane that is desorbed from the coal surface by differential adsorption of carbon dioxide on the coal surface.
Much of the early work on recovering coal bed methane was driven by a need to reduce methane levels sufficiently to enable safe mining. More recently, deep unmineable coal beds have been utilized as a source of large volumes of methane for commercial purposes.
The primary mechanism of methane retention in coal beds is by adsorption on the coal surfaces within the matrix pore structure. This is a very different mechanism for gas storage than in conventional sandstone or limestone gas reservoirs, where free gas is compressed within the pore spaces. Within the meso and micropores of a coal bed there exists tremendous surface area on which methane molecules may be adsorbed.
Another important aspect of the coal reservoir is a set of natural fractures called cleats which form during the coalification process. The dominant cleat is referred to as the face cleat with the subordinate cleat, oriented roughly perpendicular to the face cleat, termed the butt cleat. These constitute the macroporosity of the reservoir and store a small amount of compressed gas, but are often filled with water. More importantly, however, they provide a permeability conduit through which methane can flow.
Many coalbed methane wells exhibit an unusual production profile with regard to both gas and water production rates. Initially, in virgin coalbeds, the cleats may be saturated with water. A period of water production is then required prior to gas production.
The movement of gas into the cleat system eventually results in two phase flow of water and gas. Initially the water saturates the fracture system and the gas is adsorbed to the coal matrix. Only water is flowing in the cleats. As the pressure declines and the cleats are partially dewatered, gas desorption occurs. Mostly water moves in the cleats as the gas slowly starts to move in the system. The gas saturation needs to exceed critical saturation before two phase flow happens in the fracture or cleats. Diffusion of gas, after desorption from the matrix, will continue to move the gas in the fracture, and two phase flow happens around the wellbore.
As a result of this mechanism, the gas production will typically lag the water production. As the pressure is reduced, the gas desorption rate will increase causing the gas production to reach a peak, after which it will decline as the gas is depleted in the drainage area of the well.
Many procedures have been proposed over the years for improving the results of conventional methane production techniques. Most of these procedures involve injection of a fluid into one or more injection wells to displace methane and recover the methane from recovery wells spaced from the injection wells.
2. Brief Description of the Prior Art
A process for removing methane from coal beds by injecting a carbon dioxide-containing fluid, ceasing injection and holding the injected fluid in the coal bed to enable desorption of methane, followed by recovery of desorbed methane through a recovery well, is described in U.S. Pat. No. 4,043,395 to Every et al. The Every et al. patent is directed to reducing methane in mineable coal seams to a safe level for mining, and indicates that continuous injection is not as effective as the periodic shut in procedure described therein.
U.S. Pat. No. 4,883,122 to Puri et al describes recovery of methane from coal beds by injection of an inert gas, such as nitrogen, that does not adsorb to the coal.
U.S. Pat. No. 5,133,406 to Puri describes a method of injecting oxygen depleted air from a fuel cell into a coal bed to increase methane production.
U.S. Pat. No. 5,072,990 to Vogt, Jr. et al describes a method of injecting hot water or steam into a coal bed to enhance methane recovery.
An article by Reznick et al entitled "An Analysis of the Effect of CO.sub.2 Injection on the Recovery of In-Situ Methane from Bituminous Coal: An Experimental Simulation", Society of Petroleum Engineers Journal, October 1984, essentially confirms the process described in the Every et al patent discussed above.
While some of the above-described procedures have been successful to a degree, there has been a continuing need for improved procedures for recovery of coal bed methane.